This invention relates to a two-way CATV system which utilizes data lines and equipment to transmit video data signals individually to terminal units.
In general, CATV system is used to transmit television programs from a central facility (hereinafter referred to merely as "a center", when applicable) to terminal units provided at homes.
A typical example of a CATV system will be described with reference to FIG. 1. In this system, a single center facility is provided with terminal units, for instance, in several tens of thousands of homes 2. The center 1 and the terminal units form one set or group. The center 1 is connected through coaxial cables to the terminal units in the homes 2. A main cable 3 extends from the center 1, and main cable amplifiers 4 and branching units 5 are provided along the main cable 3 at predetermined points. Plural branch cables 6 extend from each branching unit 5. Extension cable amplifiers 7 and taps 8 are provided along each branch cable 6 at predetermined positions. Plural auxiliary branch cables 9 extend from each tap 8 with the auxiliary branch cables 9 leading into the homes 2. Provided in each home 2 is a terminal unit 28 including a converter 10, a television set 11 and a control box 12. The end of the auxiliary branch cable 9 is connected to the converter 10 which is in turn connected to the television set 11 and the control box 12. In this fashion, the main cable from the center 1 spreads branches repeatedly connecting the center 1 to the terminal units in the homes 2.
A signal receiving antenna 13 is installed outside the center 1. The antenna 13 is connected to a demodulator 15 in a source group 14. The source group 14 includes a video disc player 16, a video tape recorder 17, a studio 18 and other such equipment as desired. Signals from the source group 14 are applied to a modulation and transmission section 19. The section 19 includes two systems. One of the two systems includes an IF modulation circuit 20, a scrambling circuit 21, and an up-converter circuit 22 while the other includes an IF modulation circuit 23 and an up-converter 24. The outputs of the up-converter circuits 22 and 23 are connected to the main cable 3. The main cable 3 is connected to a transmitter-receiver 25 which communicates with the converters 10. The transmitter-receiver 25 is connected to a computer 26 which is connected to peripheral equipment 27 such as a printer and a display unit.
The operation of the CATV system thus constructed will now be described. First, the switch of the television set is turned on and the television set is set to a predeter-designated channel which is preferably not used by local over-the-air stations. Each converter 10 receives a plurality of signals over cable 9, and the control box 12 is operated so that the frequency of a desired channel to be received is converted into that of the designated channel by the converter 10. The channels which can be selected by the control box can be classified into a group A of channels of which television signals are received over the air by the center 1 and then retransmitted without modification, a group B of channels of independent programs provided free of charge and a group C of channels for which there is a change for viewing. There are several channels in each group. Typically, selection can be made from about twenty to thirty channels.
For the group A, a television signal received by the signal receiving antenna is demodulated by the demodulator 15 and then applied to the modulation and transmission section 19. The signal is modulated by the IF modulation circuit 23, and the frequency of the signal thus modulated is increased to a predetermined value by the up-converter circuit 24. Thus, the received television signal is changed into a signal having a desired channel frequency which is then transmitted through the main cable 3, the branch cables 6 and the auxiliary branch cables 9 to the television sets 11.
The independent programs of group B are, for instance, weather forecast programs and news programs. A television signal provided by the video disc player 16 or the video tape recorder 16, or a television signal from a live program produced in the studio 18, is modulated and frequency-multiplied by the IF modulation circuit 23 and the up-converter circuit 24 into a television signal occupying a desired channel which is then applied to the main cable 3. Each subscriber can receive the independent televison signals for a basic monthly charge, and there is no additional change regardless of the frequency or duration of viewing by the subscriber.
The chargeable programs of group C are typically new movie programs, special programs, or the like. A television signal provided by the video disc player 16 or the video tape recorder 17, or a television signal from a live program produced in the studio 18, is modulated by the IF modulation circuit 20. A scrambling synchronizing signal of predetermined form is added to the video signal portion of the television signal by the scrambling circuit 21. Because of this signal, if the television signal is received only as it is by the television set, it is impossible to reproduce normal pictures on the television set. The television signal thus treated is applied to the up-converter 22 where its frequency is increased to the frequency of a designated channel. The television signal thus processed is supplied to the main cable 3.
Upon receipt of the television signals by the television set in each home 2, the scrambled television signal is converted into a normal video signal by the converter 10 so that normal pictures can be observed on the television set 11. Whenever a chargeable program is received, a predetermined fee is charged to the subscriber, and the sum of the monthly basic charge and the special use fee is then billed to the subscriber.
In order to determine the fees owed by each subscriber, it is necessary to detect which subscribers have used what channels for particular periods of time. For this purpose, the transmitter-receiver 25 outputs a retrieving signal at predetermined time intervals to address the converter 10 in each terminal unit with an address number assigned to that particular terminal unit to determine if a group C channel is being used at the retrieval time. In response to this, the converter 10 sends to the transmitter-receiver 25 an answering signal representative of the channel which is in use at the retrieval time. The charge data transmitted and received by the transmitter-receiver 25 is arranged and stored by the computer 26 and displayed or printed out by the peripheral equipment 27. As the retrieving signal is transmitted at fixed predetermined time interval of typically several seconds to several tens of seconds, audience ratings can also be immediately calculated.
A subscriber can choose which programs he wishes to view by operating his control box 12 while watching the television set 11. The subscriber's choice or answer is encoded and sent through the coaxial cables to the center 1.
As a result of recent developments in two-way communication techniques, a communication system has been proposed in which, in response to command signals from the terminal units, the center transmits requested data to the terminal units via a television signal frequency, so that the requested data is reproduced on individual receivers of subscribers through a particular channel in the same system. However, this type of communication system has been disadvantageous in that particular television channels must be used exclusively for the transmission of data, and accordingly the number of ordinary television programs which the subscribers can enjoy is decreased.